Many ores require significant processing after having been mined. Generally, this is for the purpose of extracting the particular metal or other component of the ore, known as the "value", which has commercial value in its purified form. One technique for extracting the value is known in the art as "froth flotation". This technique is generally carried out by constructing a series of tanks, or "cells", through which the process liquid flows. For successful froth flotation, it is required that the level of ore process liquid in the cells remains at a predeternined amount while process liquid flows from the most upstream cell to the most downstream cell. The process liquid may contain significant solid pieces of one which can be abrasive to the insides of the cells and process liquid conduits.
Typically, cells are connected in groups of 2, 3, 4 or more and these groups are known as "banks". Each bank is generally separated from the next bank downstream by some means for controlling the flow of process liquid from the last (or most downstream) cell in the upstream bank to the first (or most upstream) cell in the following downstream bank. The downstream bank is physically located at a lower level than the upstream bank to facilitate the flow of process liquid by gravity.
Traditionally, a "connection box" is provided between two banks. A similar component, known as a "discharge box", is located downstream of the last bank of cells. Pulp flows from the last cell in the first bank to the first cell in the second bank through the connection box. The connection box is fluidly connected to both the last cell in the first bank and the first cell in the second bank. Generally, the inlet for the connection box is adjacent to the base of the last cell in the first bank and the outlet of the connection box is in the side wall of the first cell of the downstream bank. A dart valve is usually arranged in a port in a horizontal partition within the connection box. This partition separates either a pair of adjacent chambers defined by a vertical baffle in the connection box, or a larger upper inflow chamber and a smaller lower outflow chamber defined by the partition.
Discharge boxes are generally of similar construction and are used to control the amount of process liquid in the most downstream bank of cells, by controlling the amount of fluid permitted to flow through the discharge box.
The aforementioned vertical baffle, or a side plate separating the connection box from the downstream cell, may include an upper level slot to permit an amount of process liquid or froth to flow from the upstream bank to the downstream bank and bypass the valve. This slot is often referred to as a "weir". Thus, if there is a sudden surge in the volume of process liquid for which the valves cannot be adjusted quickly enough (for proper operation of the row of cells), the extra process liquid can flow through the weir to He next downstream bank of cells. Such weirs arc particularly necessary where the valve control mechanisms are manual, or otherwise slow to react.
A dart valve essentially includes a conically shaped valve member (the "dat") which caa be inserted into or withdrawn from an elastomer lined orifice plate or grommet. With dart valves used in connection boxes, the valve member is generally a solid elastomer to minimise wear. The valve member is connected by a connecting rod to an actuator, which is typically pneumatic.
Apart from dart valves, it is also known to use an elastomer lined "pinch" valve between the ends of banks of cells. These valves are relatively inexpensive and do not require connection boxes but do have a tubular component extending between the cells. These valves have a relatively short working life as the elastomer lining suffers from solids in the process liquid grazing the lining as the process liquid flows through the valves. Further, these valves generally have a relatively narrow range of operation and a non-linear response to adjustment. Thus, pinch valves do not require use of a connection box but can be difficult to adjust accurately, and therefore are difficult to use. This is significant because the level of process liquid in a cell needs to be kept to within a very narrow range (such as 6 millimeters in some installations).
FIGS. 1 and 2 show a typical known construction of a connection box A between two cells B and C, cell B being the last cell in the upstream bank and cell C being the first cell in the downstream bank. In FIG. 1, the connection box is an "up-flow" connection box, with the dart valve D in partition E between chambers F, G defined by baffle H with weir I. When dart valve D is open, process liquid flows through opening J at the bottom of cell B, upwardly through the open port in partition E, and through side opening K into cell C. In contrast, FIG. 2 (in which like parts are indicated by like primed reference letters) illustrates a "down-flow" connection box A' in which process liquid flows downwardly through the port in partition E' controlled by dart valve D'. Partition E' separates upper and lower chambers F, G' of the connection box. FIG. 3 illustrates a known arrangement using a pinch valve L" between cells B" and C" having openings J" and K" respectively.
More recently, ore processing plants have been built with very large cells, greater than 40 cubic meters volume, as opposed to the older plants which have cells of around 10 cubic meters. Accordingly, connection and discharge boxes have correspondingly increased in size and their construction is now a significant component of the cost of supplying a new row of cells.
It is an object of the invention, at least in the particular application under discussion, to provide a row of flotation cells at a lower cost while consistently achieving the required process liquid level.